1. Field of the Invention
The present invention relates to an apparatus for use while completing a subterranean hydrocarbon producing well. More specifically, the invention relates to an apparatus for the staging of cement between casing and a wellbore.
2. Description of the Related Art
When completing a subterranean well, casing is typically inserted into the wellbore and secured in place by injecting cement within the casing. The cement is then forced through a lower end of the casing and into an annulus between the casing and wellbore wall. A wiper plug is typically used for pushing the cement from the casing. A displacement fluid, such as water, or an appropriately weighted mud is pumped into the casing above the plug, the pressurized fluid serves as a motive force to urge the plug downward through the casing to extrude the cement from the casing outlet and back up into the annulus. However, as wells are increasingly being drilled deeper, the hydraulics for cementing the casing wellbore annulus in a substantially deep well makes the single stage cement injection process impracticable. Also, in some instances it is impossible to cement the entire well. For example, cement is not provided in portions of the well, where the well formation pressure is less than well hydrostatic pressure, or where the formation is too porous so high cement slurry pressure in the case induces formation breakdown, which leads to losses in the formation, as a result, no cement is present.
To overcome the problems of a single stage cement process, the casing string is cemented in sections, which is known as a staging process. Staging involves placing cement staging tools integral within the casing string; the staging tools allow cement to flow downward therethrough to a lower section of the casing string during primary or first stage cementing operations. When the portion of the casing string below the particular staging tool is cemented to the well, the staging tool selectively closes its bore and opens a side port to divert cement into the surrounding annulus where the cement can flow upwards in the annulus. The cement staging tools also are equipped with packers for sealing the annular area between the tool and wellbore. However, presently known tools experience failures such as failure to inflate the packer element, failure to open ports, failure to close ports, and disconnection of the tool from the casing string.